Generally, in a data processing system, data processing units are connected to a central control unit via a bus over which bytes can be transmitted from the central control unit to the processing units and vice versa. These bytes comprise a number M of bits, so that the bus must comprise M lines to transfer the M bits from the processing units to the central control unit.
In some systems, the bits sent over each line are represented by a voltage level that is either high or low during a time interval t, in accordance with the NRZ coding method. However, the NRZ method, while simple, requires that resynchronization means be provided to permit retrieving the bits at the receiving end.
In such a context, provision must be made in the central control unit for an interface device that is capable of resynchronizing the bits received from the data processing units and of identifying the units requesting access to the bus.
In addition, each unit must include a bus interface device for resynchronizing the bits received from the central control unit.
Whenever a unit desires to access the bus, it sends a Request Bus bit over a line specifically allocated thereto in the bus. For example, unit 0 will send the bit over line 0, unit 1 will use line 1, etc. Consequently, the larger the number of connected units, the larger the number of lines required will be; alternatively, a more complicated exchange protocol should be used, for example one that would involve encoding the requests placed on several lines (and employing a suitable algorithm to prevent contentions).
The disadvantage of using a bus comprising a large number of lines is that it will increase the cost of the system. Also, since most of today's circuits are miniaturized, the space taken by bulky interconnection cables and input/output connectors will obviously create packaging problems. In addition, when a great many lines are used, it is difficult to avoid variations in line characteristics (such as impedance and signal propagation time).
To develop a more complicated exchange protocol would require the use of decoding and decision-making circuits to determine which unit is to gain access to the bus, a time-consuming process.